Distributed energy resources (DERs), also commonly referred to as distributed generation, distributed energy, and distributed power systems, are small, modular, decentralized, grid-connected or off-grid energy systems located in or near the place where energy is used. DERs are integrated systems that can include effective means of power generation, energy storage, and delivery. DERs include systems such as reciprocating engines (diesel, natural gas, dual-fuel, etc.), combustion turbines, micro-turbines, fuel cells, photovoltaic systems, concentrating solar systems, wind energy systems, small modular biopower systems, energy storage systems (e.g. flywheels), etc.
Many factors including environmental concerns, system expansion constraints, and technology enhancement in DERs have led to the progressive penetration of DERs in power distribution systems. DERs are able to provide energy at customer sites, and introduce new challenges in the optimization of power distribution system operation. In particular, many DERs have the capability to control their reactive power (var) output within a certain range, which offers new possibilities to control the reactive power flow in the electric power distribution network.
Reactive power flow is needed in an alternating-current power distribution system to support the transfer of real power over the network. The portion of power flow that is temporarily stored in the form of magnetic or electric fields, due to inductive and capacitive network elements, and then returned to the source, is the reactive power. Inductive devices absorb reactive power from the network, and capacitive devices inject reactive power into the network. DERs can absorb or inject reactive power, depending on operating conditions. Inductive devices, capacitive devices and DERs connected to a power distribution network are referred to herein as reactive power resources. Energy stored in reactive power resources gives rise to reactive power flow. Reactive power flow strongly influences the voltage levels across the network. The voltage levels and reactive power flow must be carefully controlled to allow a power distribution system to operate within acceptable limits.
In power distribution systems, power loss reduction is an important solution for improving system operation efficiency. Power loss reduction involves the control of available reactive power resources to optimize the reactive power flow in the network. The predominant reactive power resources available in traditional power distribution systems are switchable shunt capacitor banks. Most conventional power loss reduction solutions therefore focus on the control of the capacitor banks only. However, with the increasing penetration of DERs in power distribution networks, additional reactive power resources are available other than just switched capacitor banks. Compared to the discrete control of capacitors, the continuous control capability of DERs can further facilitate power loss reduction. Conventional power loss reduction solutions that do consider DERs in the power loss reduction analysis are based on very simplified network models and are verifiable only for small systems. It is difficult to predict the effectiveness of such solutions on a realistic utility-scale system.